The present invention relates to an illumination control arrangement for controlling the amount of light emitted by a fluorescent lamp.
Such arrangements for controlling the amount of illumination emitted by a source of light is typically called a dimming control arrangement. Incandescent lamps have typically been dimmed either by using a diode (full illumination without the diode in the circuit and reduced illumination with the diode in the circuit) or a rheostat. Fluorescent lights have been tyically dimmed in the prior art by the use of either phase turn on or phase turnoff control.
In phase turn on control, the current supplied by an alternating current source is kept off until a predetermined point in the alternating current cycle is reached at which time current is allowed to flow to the fluorescent light until the next zero point in the alternating current cycle. By adjusting this predetermined point, the amount of light emitted by the fluorescent light can be controlled.
In phase turn off control, current is supplied to the fluorescent light beginning with the zero point in the alternating current cycle but is turned off at some predetermined phase and held off until the next zero point in the alternating current cycle. Again, by adjusting the predetermined phase for turn off, the amount of light emitted by the fluorescent light can be controlled.
The prior art, as shown in U.S. Pat. No. 4,350,935, also discloses a notch control arrangement for a fluorescent light wherein current is supplied to the light beginning with the zero point in the alternating current supply voltage, turned off at a first predetermined phase, and turned back on at a second predetermined phase after which the alternating current is allowed to flow to the load through the next zero point in the alternating current supply voltage until the next phase turn off point is reached. The width of the notch is adjusted for controlling the amount of light emitted by the fluorescent light.
Dimming of fluorescent lights over a wide range has proven elusive. The excitation of the molecules of the gas within the fluorescent light causes the gas to emit light. Dimming is achieved by controlling the ionizing current through the fluorescent light to control the amount of excitation of the gas molecules. In order to excite the gas molecules within the light, however, the filament/cathodes are heated by electric current to reduce the work function for their emmission of electrons and allow the voltage gradient to develop in the gas so the gas molecules can become ionized. One object of prior art fluorescent light dimming arrangements is to provide a control circuit which maintains the filament/cathode voltage high enough to maintain a low work function to emit electrons while at the same time controlling the amount of ionizing current to thus control the amount of emitted light. However, these prior art arrangements have been unable to dim fluorescent lights to very low levels. The present invention is able both to achieve control of the ionizing current without unduly sacrificing filament/cathode voltage and to achieve a wider range of illumination control by controlling the position of a notch in the line voltage waveform.